Light sources for digital pen

ABSTRACT

Motion of a writing instrument is tracked from sensors located in the vicinity. The signals generated from the sensors are processed and used in a wide variety of ways.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/623,284, filed Jul. 17, 2003; which is a continuation of U.S.application Ser. No. 09/698,471, filed Oct. 27, 2000, now abandoned,which claims the benefit of U.S. Provisional Patent Application Ser. No.60/161,752, filed Oct. 27, 1999, U.S. Provisional Patent ApplicationSer. No. 60/195,491, filed Apr. 10, 2000, and U.S. Provisional PatentApplication Ser. No. 60/230,912, filed Sep. 13, 2000, and is acontinuation-in-part of Ser. No. 09/376,837, filed Aug. 18, 1999, nowU.S. Pat. No. 6,577,299, which claims the benefit of U.S. ProvisionalPatent Application Ser. No. 60/142,201, filed Jul. 1, 1999, U.S.Provisional Patent Application Ser. No. 60/142,200, filed Jul. 1, 1999,and U.S. Provisional Patent Application Ser. No. 60/096,988, filed Aug.18, 1998.

The disclosures of the prior applications are considered part of and areincorporated by reference in the disclosure of this application.

BACKGROUND

This invention relates to tracking motion of a writing instrument.

By tracking the motion of a pen, for example, as it is used to write ordraw on paper, it is possible to capture and reproduce electronicallywhat is being written or drawn. Motion of a stylus that does not leave amark on a writing surface can also be tracked.

In some proposed approaches, the surface on which the pen is moving mayhave an array of pixels or other sensing locations each of whichresponds when the pen is at that location.

In other techniques, the pen tracking is done entirely by electronicsmounted in the pen. In some schemes, the moving pen communicates withstationary sensors that are separate from the pen, and triangulationalgorithms are used to track the motion.

SUMMARY OF THE INVENTION

In general, in one aspect, the invention features a method that includesconveying light from a moving writing instrument as an indication of thelocation and path of the writing instrument on a two dimensional writingsurface; sensing the light at two or more sensors and generating asequence of signals representative of the sensed light; and applying atechnique to reduce the effect of variations of the light intensity in athird dimension with respect to the generated signals.

Implementations of the invention may include one or more of thefollowing features. The technique may be based on optics that areconfigured to enhance the uniformity of signal response of the sensors.The lens may be a spherical lens or an aspheric lens. The sensors may bearrays of sensitive pixel elements or analog sensors. The technique maybe based on algorithmic processing of the generated signals. Thealgorithmic processing may include linearizing the signal response ofthe sensors based on parameters associated with the writing instrument.The technique may be implemented in digital hardware or in analogcircuitry. The algorithmic technique may reduce the effect of variationsof the light intensity based on other than dimensional effects. Thesignals may be grouped in frames, and the signal processing techniquemay include processing of multiple frames to cancel noise. The lightconveyed from the moving writing instrument may be modulated at afrequency related to the rate at which the signals are generated by thesensors, and the sensor signals may be chopped at the frequency ofmodulation. Opposite gains may be applied to each of the chopped signalsdepending on the on or off state of the light conveyed from the writinginstrument that corresponds to the signals. The frame rate may bevaried. The chopped signals may be integrated over time. The lightconveyed from the writing instrument may include a strong short pulseimposed at on the modulation frequency, a phase lock loop may determinethe modulation frequency from the sensor signals, and the sensor signalmay be sampled at the times triggered by the phase lock loop during theduration of the strong short pulse. The characteristics of the conveyedlight may be used for synchronization between the writing instrument andthe sensors. The conveyed light may include periods of lower frequencymodulation and bursts of higher frequency modulation, and the sensorsignal associated with the higher frequency bursts may be used to lockonto a modulation clock.

In general, in another aspect, the invention features a method thatincludes conveying light from a moving writing instrument in atime-changing pattern of directions, sensing the light at two or moresensors located at two or more different locations spaced from thewriting instrument, and determining the location of the writinginstrument by detecting a phase difference between signals measured atthe two or more sensors.

Implementations of the invention may include one or more of thefollowing features. The time-changing pattern of directions may includea rotating pattern with respect to an X-Y plane on which the writinginstrument is moving. The signal radiated in the positive X directionmay be in phase quadrature to the signal radiated in the Y direction.

In general, in another aspect, the invention features apparatus thatincludes sensors configured to receive light from a writing instrumentmoving across an X-Y writing surface, the light having variations inintensity along a Z-axis normal to the writing surface, and opticsconfigured to enhance optical power of the light received from thewriting instrument.

Implementations of the invention may include one or more of thefollowing features. The optics may be a ball lens or an aspherical lens.The optics may include a single spherical lens and the lens and thecorresponding sensor may be together configured to enhance the opticalpower of light received at large angles or longer distances or atdisadvantageous positions of the writing instrument. The optics mayinclude a special lens configured to enhance optical power of the lightreceived from a location on the X-Y surface that is beyond apredetermined position. The optics may include two cylindrical lenses,one nearer the sensor to project light horizontally onto sensor, and theother positioned to collect light in the Z-axis dimension, the otherlens having a body that is bent around the first lens. The algorithmicprocesses may enhance the immunity of the signals to variations in theintensity of the received light caused by distance from or tilt of thewriting instrument. The processes may determine the integral power ofthe overall signal distribution on the sensor and calculate a subpixelposition based on half of the integral power position. The processes mayuse a polynomial approximation on the signal distribution and calculatea subpixel position as a position of approximated maximum. Thecalibration procedure may produce parameters to be used in combinationwith data from the sensors. The calibration parameters may correct formanufacturing deficiencies of the optics and the sensors, and thealgorithmic processes may use a straight triangulation technique todetermine a position of the writing instrument. The calibrationparameters may correct for manufacturing deficiencies of the optics andsensors and the algorithmic processes may determine the position of thewriting instrument using polynomial series, where coefficients in thesepolynomials are determined during the calibration procedure.

In general, in another aspect, the invention features a method thatincludes receiving light from a moving writing instrument at a an arrayof sensing elements of a sensor, reading the sensing elements insequence to generate a sequence of signals indicative of light sensed bythe elements of the array, and resetting each of the elements after itis read and before at least some of the other elements in the array areread.

Implementations of the invention may include one or more of thefollowing features. The array may include a CMOS or CCD position sensor.All of the elements may be read before all of the elements are reset.

In general, in another aspect, the invention features a method thatincludes determining a sequence of three-dimensional positions of themoving writing instrument based on the signals.

In general, in another aspect, the invention features the combination ofa writing instrument having an elongated housing configured to behand-held, a light source in the housing, and a lens in the housingconfigured to receive light from the light source and to convey thelight through a free-air path to optical sensors spaced from the writinginstrument, the lens being configured to be semi-reflective.

In general, in another aspect of the invention, the light sourceincludes an array of light sources arranged around an axis of thewriting instrument and configured to emit light in a direction normal tothe axis.

Implementations of the invention may include one or more of thefollowing features. The lens may be configured to internally reflect andconcentrate the light and to emit it by reflection from a reflectiveexternal surface of the lens. The lens may have a cylindrical bodyhaving an upper surface that receives the light and a lower annularsurface that reflects the light toward the optical sensors. Thereflective external surface may include a conical surface oriented at a45 certain degree angle to a longitudinal axis of the writinginstrument. The light source in the pen may include LEDs arranged in aring.

In general, in another aspect, the invention features a deviceconfigured to turn the light source on and off in response to a userapplying pressure from the writing instrument to a writing surface, thedevice being configured so that an amount of pressure required totrigger the device is not so large as to disrupt normal writing motionof the writing instrument on the writing surface.

Implementations of the invention may include one or more of thefollowing features. The writing instrument may include a ballpointcartridge having a writing point and the device may be positioned at theopposite end of the cartridge from the writing point. The device may bea switch or a pressure sensor.

In general, in another aspect, the invention features a holder having areceptacle for receiving at least a portion of the writing instrumentfor storage of the writing instrument,

the writing instrument and the holder containing respective elementsthat enable wireless transmission of signals associated with motion ofthe writing instrument and tracking of the writing motion based on thesignals.

In implementations of the invention the holder may be a pen cap and mayinclude a clip configured to attach the holder to a stack of pages or toa notebook. The holder may include at least two light sensors and aprocessor that processes signals from the light sensors to determine asequence of positions of the writing instrument. The holder may includea receptacle for holding the writing instrument and for enablingrecharging of batteries in the writing instrument.

In general, in another aspect, the invention features an element thatenables wireless transmission of a signal associated with motion of thewriting instrument and tracking of the writing motion based on thesignal, the element being built into a cell phone, a PDA, a webpad, or aclipboard.

In general, in another aspect, the invention features, a holder that hasa mechanism for attaching the holder to a writing substrate in anorientation that enables the elements to be used in conjunction with thewireless transmission. The clipping mechanism may include a switch toactivate functions of a processor in the holder when the clippingmechanism is manipulated.

In general, in another aspect, the invention features, a holder thatincludes a receptacle for the writing instrument and a rechargingcircuit connected to recharge the battery when the writing instrument isin the receptacle.

In general, in another aspect, the invention features a CMOS sensoradapted to receive light associated with motion of a writing instrumentand to provide signals indicative of an angle of receipt of the lightwith respect to a known direction, and a lens aligned to direct thereceived light to the CMOS array.

In implementations of the invention, the lens may be optimized forcollection of light from an area in which the motion of the writinginstrument occurs. The lens may be a field lens or a Fresnel lens. Thelens system may be configured to collect light in a dimension normal toa plane of motion of the writing instrument and to project the lightonto the sensor in a direction parallel to the plane of motion.

In general, in another aspect, the invention features calibrating bypositioning a writing instrument at a succession of positions on awriting surface, generating signals at sensors from light received fromthe writing instruments at the succession of positions, and determiningcalibration parameters for the writing instrument for use in calibratinga process that determines the positions of the writing instrument as itis being moved.

In implementations of the invention, the calibration parameters mayinclude coefficients used in polynomial series that are part of theposition determining process.

In implementations of the invention, the positions do not lie on aregular rectangular grid.

In general, in another aspect, the invention features (1) identifyinglocations on a writing surface that correspond to input elements to beentered into an electronic device, the writing surface beingnon-electronic and separate from the electronic device, (2) using awriting instrument to point to selected ones of the identified locationscorresponding to input elements to be entered, and (3) sensing thelocations at which the writing instrument is pointing and entering thecorresponding data into the electronic device.

In implementations of the invention, the writing surface includes asheet of paper, the input elements comprise characters of language orcommands that are printed on the writing surface.

In general, in another aspect, the invention features moving a writinginstrument across a non-electronic writing surface to indicate a path,and remotely sensing the path and generating signals for use in enteringthe path into an electronic device that is separate from the writingsurface.

In general, in another aspect, the invention features modulating lightthat is conveyed from a moving writing instrument to light sensorsspaced from the writing instrument at a predetermined frequency, andusing a phase locked loop associated with the sensors to lock onto thephase of the modulated light.

In general, in another aspect, the invention features, circuitry fortracking writing motion of a writing instrument using wirelesstransmission of signals between the writing instrument and a stationaryelement, the stationary element including a main processor and aseparate preprocessor, the preprocessor being connected to perform atleast data capture with respect to motion of the writing instrument, themain processor being connected to perform at least data communicationwith respect to the tracking.

In implementations of the invention the preprocessor may also beconnected to perform user interface functions and sub-pixel data storageand the main processor may also connected to perform backgroundcancellation and sub-pixel calculation.

In general, in another aspect, the invention features a reflectiveelement configured to reflect light received from outside of the writinginstrument to the sensor for use in tracking motion of the writinginstrument.

In implementations of the invention, the reflective element may reflectlight to the sensor when the writing instrument is being used forwriting and disable the reflective element from reflecting light to thesensor when the writing instrument is not being used for writing.

In general, in another aspect, the invention features receiving lightfrom a moving writing instrument at a light sensor having an array ofsensitive pixel elements, and determining the location in the array atwhich the maximum intensity of light has been received from the writinginstrument, the location being determined with sub-pixel accuracy.

In implementations of the invention, the sub-pixel location isdetermined by determining the integral pixel location that is closest tothe subpixel location, and finding a fractional center of gravity of asubarray that is centered on the integral pixel location.

In general, in another aspect, the invention features indicatinglocations on a non-electronic surface that correspond to inputs to anelectronic device, and detecting the locations and inputting them intothe electronic device.

In general, in another aspect, the invention features a clip forclipping paper on which the writing instrument is to be moved to thesensor.

In implementations of the invention the mechanism may be part of aclipboard or a notebook, the clip may include a mechanism that enables auser to cause the clip to grip or to release the paper. The clip mayinclude an activation button and a spring and a lever operated by thebutton. The lever may be configured to rotate in response to the button.The button may be configured to be pushed or pulled. Other advantagesand features will become apparent from the following description andfrom the claims.

BREIF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows pen tracking.

FIG. 2 shows a pen.

FIG. 3 shows a lens in a pen.

FIG. 4 shows a lens in a pen.

FIGS. 5 and 6 show reflection of light in a pen.

FIG. 7 shows a tracking method.

FIG. 8 shows a pen.

FIG. 9 shows a pen.

FIG. 10 shows a holder.

FIG. 11 shows a lens in front of a sensor.

FIG. 12 shows a holder.

FIG. 13 shows a holder.

FIGS. 14 and 15 show half of a holder.

FIG. 16 shows another half of the holder.

FIG. 17 shows a field of view.

FIG. 18 shows a block circuit diagram.

FIG. 19 shows a state diagram.

FIG. 20 shows a timing diagram.

FIG. 21 shows a timing diagram.

FIG. 22 shows geometry of tracking.

FIG. 23 shows a circuit diagram.

FIG. 24 shows a timing diagram.

FIGS. 25 and 26 shows a rotating beam technique.

FIG. 27 shows a channel diagram.

FIG. 28 shows a channel circuit.

FIG. 29 shows a paper keyboard.

FIG. 30 shows a spherical lens.

FIG. 31 shows an aspherical lens.

FIG. 32 shows a two-lens arrangement.

FIG. 33 shows a clip.

FIG. 34 shows a clip.

FIG. 35 shows a sliding belt clip.

FIG. 36 shows two views of a clip.

FIG. 37 shows two views of a clip.

DETAILED DESCRIPTION OF THE INVENTION

We describe an electronic wireless pen that in addition to its regularfunction of leaving a visible trace on the writing surface also emitsinfrared (IR) light that is collected by external IR sensors to measurepen position with respect to the sensors. The sensors are CMOS or CCDlinear or 2D arrays, Position Sensitive Detectors (PSD) or other lightsensitive detectors. The sensors can be clipped to the edge of writingsurface allowing reconstruction of writing on that page. The position ofthe pen is determined by mapping the sensor reading to the actual XYposition of the pen on paper.

This electronic input device looks like a regular pen with a holder. Theuser writes with it just as with any ordinary pen on paper, notebook orany other flat surface. It is used to capture handwriting text ordrawings. The pen stores all its movements during its use by recordingsensor measurements into its memory. The pen then downloads it to acomputer, personal digital assistant, handheld computer or cellularphone. The handwriting, as it appears on a page, is then automaticallyreconstructed from sensor information.

As shown in FIG. 1, a pen or other writing instrument 10 that leaves avisible trace 12 of writing or drawing in the usual way on a sheet ofpaper or other writing surface 14 may also have a source 16 that emitsinfrared (IR) light 18 for use in automatically tracking the motion ofthe pen. The light is detected by IR sensors 20, 22 that are heldstationary relative to the pen at a nearby location, for example, nearthe edge 23 of the paper.

The sensors deliver sequences of signals that represent the position ofthe pen on the writing surface (e.g., angle 24) at which the light isreceived from the pen for each of a succession of measurement times.Circuitry associated with the sensors uses an algorithm to process thedirectional information (and the known distance 26 between the sensors)to determine a succession of positions of the pen as it is moved acrossthe writing surface. The algorithm can use a mathematical model thattranslates pixel signals of the sensors into positions on the writingsurface. The algorithm could be a quasi-triangulation algorithm usingcalibrated parameters (distance from lens to sensor and horizontaloffset between their centers of refractive index) or it could be apolynomial approximation.

The tracked motion of the pen can be used to recognize handwriting orcapture drawings created using the pen or used in a wide variety ofother applications. The tracked motion information can be sent to alocal personal computer or to a central computer through a personaldigital assistant, a handheld computer, or a cellular phone for centralstorage and processing.

Tracking of Light Source with a Two or One Dimensional Sensor

The problem of tracking XY bearing of a pen can be formalized asfollows.

The pen carries a finite source of light close to the tip. This sourceemits light which intensity in the test point depends on the XYZposition of a test point with the source in the origin.

A multichannel detector is located at another location. It collects someportion of the light emitted by the source on a pen. Intensity deliveredto different channels varies depending on the XYZ position of channelinputs with respect to the location of this source. Intensity data aresufficient to calculate the XYZ position of a source relative to thedetector. Intensity data are also subject to noise including sourceinstability, detector noise, and other kinds.

We are interested in obtaining the XY bearing of the pen only. In fact,all three coordinates will vary due to thickness irregularities on awriting surface and due to varying tilt of a pen during writing. Alongwith noise this will cause complex variations in channels reading.

There are different ways of processing such signals: weighted average(center of gravity), median computation, thresholding, etc. They mostlyaddress noise cancellation and treat Z motion of a source as noise also.

Our goal is to establish such a property of the detected signal thatwould be invariant to the motion of a pen in Z direction and to somesources of noise.

For this purpose, we introduce an aperture between a detector and thesource. This aperture may contain a lens, for example. Thus we obtain aspatially limited signal. This means that there is a closed group ofdetector channels that is excited by both the signal and the noise (asegment in case of a linear array detector). This group is surrounded bychannels that are excited by the noise only. In the absence of anaperture, all channels are excited by both the signal and the noise.

After creating such a signal we establish a specific point (e.g.maximum) and define a processing window around this point in such a waythat it extends beyond a spatially limited signal. Then a cumulativedistribution function of data inside the processing window is calculatedversus channel numbers. The projection of this function's half magnitudepoint on channel numbers produces the invariant property. While channelnumbers are integers, the invariant property value may be fractional.

There are basically two types of detectors, whether 2D or 1D. Eachdetector can be a two channel detector, like a PSD, or a “multichannel”detector like a CMOS and CCD device. PSD detectors have two outputsignals whose ratio defines a relative position of incident light spot.CMOS and CCD detectors have a number of pixels. Each pixel defines aparticular space on the detector and has an analog output. These analogoutputs can be digitized for later processing in firmware or software,or can be processed by analog means. Algorithms used in software canalternatively be implemented in hardware in the same way.

One Calibration Procedure

There is no need to achieve a linear response of the detectors to penmotion as has been proposed in other known approaches. A linear responsewould be required if simple triangulation were to be used to interpret adetector reading as an XY position of a pen.

An unambiguous dependency exists between the XY position of a pen andleft and right detector (L and R) readings as follows:X=Fx(L,R);Y=Fy(L,R).  (1)

These functions can be expressed as polynomial series. Coefficients inthese polynomials can be determined during the calibration procedure.

During the calibration procedure, the pen is positioned in differentknown XY locations on the paper and readings of both detectors are takenand stored for future processing. After a sufficient number of pointshas been accumulated, common linear algebra methods are used tocalculate the coefficients in (1).

We know that system (1) is substantially non linear. It is important tolocate calibration points in such a way that a necessary resolution isachieved across the entire writing area. We do not locate thecalibration points in the nodes of a regular rectangular grid. Instead,we use mathematical models to match a calibration grid to particularnonlinear properties of our detectors.

Another Calibration Procedure

If simple triangulation is used to calculate an XY pen position fromdetector data, we intentionally introduce an error into the geometricalparameters of our detectors to allow for the nonlinear propertiesdiscussed above.

We know exactly the refractive index of the lenses in our detectors anddistances between the lenses and sensors by virtue of our design. At thesame time it has been proved that varying these values in triangulationcomputation one can effectively compensate the nonlinear properties ofdetectors.

To obtain effective values for refractive index and distances betweenthe lenses and sensors we run another calibration procedure. Thedifferent XY location calibration points are necessary for properresolution across the writing area. Locations of calibration points forthis disturbed triangulation are obtained through mathematical models.

The Pen

As shown in FIG. 2, in one example, the IR source in the pen can be anLED 13 that emits IR light 15 at the tip 17 of the pen when pressure isapplied during writing. In this example, the LED source 13 is formed bya ring of LEDs 19 arranged around the longitudinal axis 21 of the pen(only two LEDs are shown).

Light from the LEDs is project downward toward the pen point and into abody/lens of acrylic material 18 that operates as a light pipe. Theacrylic lens diffuses and transmits the received light so that lightemitted from the pen is delivered along optical paths in all directionsaround the pen.

As shown in FIG. 3, the pipe 18 is polished and reflective andconcentrates light 502 from the LEDs 19 inside by not allowing the lightto escape sideways. The bottom part of the pipe is also polished at the45 degree conical surface 504 at the bottom of the pipe. A reflectivecylindrical shell 506 helps to confine and cause mixing of the lightthat is emitted from the LEDs. A conical body 508 supports the lightpipe. Downwardly directed light within the pipe is reflected from theconical surface 504 and delivered to the air at all angles around thepen.

FIGS. 5 and 6 illustrate side and top views of internal reflection oflight in the light pipe. Most light from the LEDs passes along thelength of the body of the pen and is reflected at a 90 degree angletoward the sensors. Some other light finds its way out of the pen atangles different from 90 degrees.

As shown in FIG. 2, the light that is emitted from the pen is confinedto a vertical space 11 that is near to the writing surface 13 so that asmuch of the light as possible can reach the sensors (not shown), whichare also positioned within a small distance of the writing surface.

Other configurations having different shape light pipes/lenses could beused, including the one shown in FIG. 4, which may have a bettercoupling between the LED and the light pipe and more effective splittingand directing of light toward the reflective surface at the bottom ofthe pipe.

The pen in this example (FIG. 2) includes a ball-point pen cartridge 23that terminates in a writing point 25. When the user bears down on thewriting point during writing, a pressure switch 26 delivers a signalthat can be used to turn on the LEDs and to trigger functions ofcircuitry 28 also mounted in the pen. Circuitry 28 and LEDs 19 arepowered by a battery 30. The components are all held in a housing 15.

As shown in FIG. 7, the pen position in an x-y coordinate system 40parallel to the writing surface is determined from two angles alpha andbeta that are sensed by the two sensors 20, 22, and the known distance26 between sensors.

In another example, shown in FIG. 8, the pen is powered by threeminiature AAA-like NiCd rechargeable batteries 51 that are held in theback of the pen (For better weight distribution the batteries will bemoved closer to the tip, with the circuitry placed at the back). Thebatteries power the electronic circuitry 28 directly without anyDC-to-DC converter. The power is delivered only when the pressure switch26 is activated. The light activation switch travels only a shortdistance (e.g., 0.25 mm). The switch is preloaded by a spring mechanismto minimize refill travel, which should not exceed 0.008-0.010 inches.

Pressure sensors would be an way to effectively match pressure on thepen refill with activation of LED, as many off-the-shelf switches havean activation pressure above the desired level of 20 to 30 g.

The electronic board 28 positioned behind the battery generatesmodulation frequency pulses at approximately 100 Hz and a 50% duty cyclefor the IR LEDs along with the bursts of 1 to 10 kHz to generate pen onand pen off signals for sleep mode.

The light emitted from the pen is visible in all directions to enablethe pen to be used in any orientation in the hand. The closer theemitted light is to the tip of the pen, the less is the error due to thevariations of pen angel to paper, and the more accurate is the trackingof the tip of the pen. The LED light should be in an IR region away fromthe visible light spectrum so that ambient light from the sun and lightfixtures does not interfere excessively with the IR emission and is notvisible to the human eye.

The IR source at the pen and the orientations of the sensors in theholder are arranged to assure that as the pen tilts and rotates duringnormal writing or drawing, its IR beam reaches the sensors.

FIG. 9 shows a more detailed isometric view of a partially assembledpen.

Pen Holder

As shown in FIGS. 1 and 10, the sensors can be housed in a typical pencap 70 in which the pen can be held when not in use. When the pen isbeing used, the pen is removed from the cap and the cap is positioned ata stationary location near the writing surface and in the vicinity ofthe pen. In some examples, the sensors are linear CMOS arrays(available, for example, as 1024 pixel arrays from Photo Vision Systems,LLC (PVS) (P.O Box 509, Cortland, N.Y. 13045) (part number LIS1024)).Other linear CMOS sensors from PVS or other companies with the same or adifferent number of pixels could also be used. The analog output of eachsensor is a sequence of 1024 analog signals, one from each sensor pixel.

A shown in FIG. 10, the holder may include a clip 62 to attach theholder to the edge of a pad of paper or a notebook.

A third sensor in the form of a photodiode 56 in the middle of theholder is used (among other things) to wake up the processor from asleep mode (described below) when writing begins (e.g., the pen beginsto emit light).

The third sensor signal may also be used to synchronize the circuitry inthe pen with the circuitry in the sensor system. All three sensors arecovered by IR filtering windows that face the writing surface.

As shown in FIG. 11, in one example, the front surface 100 of each ofthe main sensors has a vertical height 104 of 125 micrometers and adistance 106 from the front surface 108 of the lens 110 of fourmillimeters. The FOV 112 is 10 degrees. The pen tip 114 directs IR lightinto the FOV when the pen tip is on the paper 116.

As shown in FIG. 17, the two sensors 88. 90 are positioned 100 mm apart.Each of the sensors has a field of view (FOV) 94, 96 centered on an FOVaxis 195, 197. The axes of the FOVs are not parallel but are toed in byan angle 199 to increase amount of overlap of the FOVs. The FOV of eachsensor has a breadth of 150° in the horizontal (x-y) plane and a heightof +/−5° in the vertical plane.

The FOVs do not cover some locations 101 on the writing surface that areclose to the edge of the paper 303, and the FOVs are arranged so thatthe dead zone 98 does not extend more than 25 mm from the holder.

In another example of a pen holder, shown in FIG. 12, the sensors 117and the lenses 119 are mounted on a holding bracket 121 with the IRfilter 123 in front. The bracket is mounted on a printed circuit board125 and is held in a housing 127. The centers of two main sensors areseparated by 100 mm.

The light from the pen is collected from two sensors in order toidentify the linear position of a modulated light source within adefined area (8.5×11 inches). The linear position may be computed bytriangulation, a lookup table, polynomial approximation, or acombination of any of these.

The sensors are flat, linear, multi-pixel sensors. Different pixels ofeach of the sensors are illuminated when the light source is indifferent locations within the field. As the light source moves acrossthe field, the corresponding movement of the light across the pixels ofthe sensors may not be linear, but the lack of linearity can be handledbecause the linear position may be computed by math and knowledge ofoptics combined with calibration data from the pair of sensors.

Instead of seeking a linear response from the sensors, we seek tomaximize the light from within writing area that falls onto the sensors.Correct reproduction of writing is achieved by using parameters savedfrom the prior calibration procedure. The system uses, in someimplementations (in the polynomial example shown below, the number ofparameters can be greater) and supplied in a separate file) only fourparameters to be passed from each pen to a host or server to process thedata and linearize it. The particular calibration parameters for a penare stored in the memory of the pen during production test andcalibration. The parameters can also be stored on the server or on a PCused by the user instead of being passed on from the pen duringdownloads.

A lens or a set of lenses accompanies each sensor. The goals of theoptical system are to maximize the efficiency of the light delivery,cover the entire field of view of the field, provide a uniform signalresponse across the entire field, and make the optical system as smalland as cheap as possible. These goals are met in part by the followingsteps:

As shown in FIG. 30, a spherical lens 753 is used to focus the light onthe sensor 755. The focal plane has a shape of a semicircle. Thedistance 759 from the lens to the sensor is optimized as are the otheroptical and mechanical properties of the lens including focal length,diameter, thickness, and material.

As shown in FIG. 31, an aspheric lens 760 may be designed with a focalpoint positioned on the sensor as the light source travels around theperiphery of our field where the total power of the light sourcedelivered to the sensor will be the weakest. Thus, this aspherical lensis designed to have a focal plane, which coincides with the plane of thesensor for only the points that are on the periphery of the page. Pointswithin the page will be out of focus, but the amount of light fallingonto the sensor will be significantly larger (closer to sensor or betterangle), and the signal stronger.

As shown in FIG. 32 (which includes a top view above and a side viewbelow), two perpendicular cylindrical lenses 770, 771 may be usedinstead of one lens. The length of the sensor limits the focal length ofthe lens in the horizontal axis. Therefore the lens diameter must besmall and the lens must be located close to the sensor. In the verticalaxis the lens may be located further from the sensor so the diameter canbe larger. The larger diameter will allow for the collection of morelight from the light source. The first cylinder (closer to the lens)will focus the light in the horizontal axis. This lens may be sphericalbecause the spot size is not very important in the horizontal axis andwill not vary that much within our given field. The second cylinder willhave power in the vertical axis. It is important to focus as much lightas possible on the sensor in the vertical dimension. For this to betrue, the light must travel an equal distance from the cylinder to thesensor for all angles. To accomplish this, the second cylinder should bebent into an aspheric shape. Any of these two cylindrical lenses can beFresnel lenses in order to save space.

In a more detailed example of a pen holder shown in FIGS. 13, 14, 15,and 16 the sensor system is held in a housing 79 that has a bottom 80and a top 82. The bottom 80 holds a clip 62 (not shown). Paper can beinserted between the clip 62 and the bottom of the pen holder when aclip button 86 is depressed. When the button is released, the clipgrasps the paper. The pen clip holds a 7 mm-thick stack of paper sheetsor a standard notepad 83. The clip positions the pen holder on the paperso that the side 87 that faces the pen is vertical with a tilt of nomore than +/−1°, thus assuring that the sensors will receive IR lightfrom the pen when it is being used to write on the surface.

The holder can also just sit on top of paper or notebook without use ofclip.

In the holder shown in FIGS. 13 through 16, the two sensors are mountedbehind IR filtering windows 89, 91, and the photodiode 93 is mounted inthe middle. An “ink well” 95 can receive the tip of the pen 97 fortemporary storage, and a tube 99 provides a place to store the pen. Thepen can be fully inserted into the tube and the batteries in the pen canbe recharged during storage.

Various mechanisms for operating the clip are possible including theexample shown in U.S. patent application Ser. No. 09/376,837, filed Aug.18, 1999.

In one arrangement, the clip mechanism shown in FIG. 33 is used. Thefigure shows the steps in operating the mechanism, as follows.

Step 1: Mechanism is not activated.

Step 2: When the push button 780 presses on a spring 782, the latterreleases the bracket 733, and lets the hinge spring 784 unfold the clip785.

Step 3: Paper 786 is inserted between the clip and the body 787 of thepen holder.

Step 4: Lever 785 (the clip) is against the paper, and spring 788, whichis significantly weaker than the hinge spring gives in and startscollapsing. The clip rotates around rotating point 789.

Step 5: The clip presses the paper against the bottom of the holder.Spring 788 collapses, and both levers move down by the amount of paperinserted.

A clip button can be used to transfer horizontal motion to verticalmotion by a lever, which lowers the clip. Or, the button can pushagainst a lever that rotates, transferring horizontal motion to verticalmovement, which lowers a clip.

This mechanism is shown on FIG. 34. The clip has two vertical slidingbars 790, 791 connected to a horizontal bar 792. There is a spring 793between the horizontal bar and the body of the holder. There are twovertical guides 794, 795 for the vertical bars to go up and down.

In the side view at the bottom of the figure, when the button is notdepressed, the spring is all the way up and the clip is pressed againstthe penholder body (left side of the figure). When the button isdepressed (right side of the figure), the spring is depressed and theclip (see the front view now), goes down between the slides. After paperis inserted and the button released, the spring pushes the clip up andthe mechanism grabs the paper between the clip and the penholder body.

The button may move a lever in multiple axes to lower the clip. Thebutton can activate a lever that rotates and moves linearly to lower theclip.

In another arrangement, shown in FIG. 36, the operator pushes down on asliding panel (button) 901. The sliding button contacts the center oftwo spring-loaded levers 903, 905 moving these fulcrums down in the samedirection of the button/panel. The bottom ends of the levers are fixedabout rotating pins 907, 909 and thus the bottom? ends move downwardabout twice the distance of the button. The top ends of the levers arefitted with pins 911, 913, which ride in guide slots 915, 917 and in twotabs that are bent up vertically from the bottom clip. The end result isa movement vertically downward of the clip, which is about twice thedistance as the button/panel travel.

Alternatively, as shown in FIG. 35, the sliding panel 930 may be placedin a horizontal orientation and, by means of a rigid flexible belt,achieve the desired result with a horizontal pushing motion as opposedto the vertical motion of the button.

As shown in FIG. 37, another approach may be achieved by a pullingmotion of the button 951 if the forces acting on the two levers 953, 955in the mechanism is moved from the middle of the levers to the bottomends, and the fulcrums of the levers are moved to a point ⅓ the distancefrom the top? ends to the bottom? ends. This would provide the necessarymechanical advantage to maintain the 2:1 ratio of the distance the clipmoves to the travel of the button.

In both mechanisms, the location of pivoting points to points ofapplying force on levers can be used to increase mechanical displacementof the clip.

Pen Holder Circuitry

A circuit block diagram of the holder is shown in FIG. 18. An ASIC 205is powered by a battery 511, or from an AC adaptor 513, or from a USBconnection to a host computer 211. The two CMOS sensors 201, 203 haveoutputs that are connected 165, 167 through operational amplifiers 169,171 through a multiplexer 180 and a 12-bit A-to-D converter to the ASIC.

The analog output of the CMOS sensor is subjected to signal processingin the form of offset cancellation and automatic gain control. Thesignal-to-noise ratio requirements of the processing implies use of 5Vpower for the CMOS sensors and all analog signal processing circuitry.Some A/D converters are operated at a 2.5v reference, and the signalfrom the CMOS sensor may scaled down by factor of 2 using a resistivedivider.

The ASIC could be model Clarity 2B from Sound Vision, located inFramingham, Mass.) and is based on an ARM7 core. The ASIC firmwareimplements data acquisition, data storage, file system management, I/Oservice (LEDs and switches), RS232, and USB communications, powermanagement for idle and sleep modes, optical calibration, and test mode.

The multiplexer enables the A-to-D converter 182 to alternate betweenthe two CMOS arrays 201 and 203 to minimize the time skew between thetwo sensors. The clock frequency of the A-to-D converter is 1.2 MHz.Each CMOS sensor is clocked at 600 kHz. Data acquisition uses the ASIC'sdirect memory access (DMA) facility.

A wakeup input of the ASIC is driven by a PLL 513 that receives an inputsignal from the photodiode 515. The photodiode is driven by modulatedlight from the pen.

The ASIC is clocked by a 48 MHz crystal and a clock divider 517. I/Ofeatures are provided through a USB port 211 and an RS232/IrDA port 209.Firmware and data are processed in SDRAM 207 and stored in a flashmemory 519. An optional LCD 172 can be provided for user display.

USB provides two data transfer modes from the holder to a PC: bulk andreal-time. Real time transfer is interrupt driven and is used forkeyboard and mouse replacement applications.

A dual function transceiver is used to implement both RS232 and IrDAcommunication. The RS232 communication is used as a dial up connectionto the server over cellular phone.

The holder can support different types of external connections,including USB, Serial, Parallel, IrDA, Bluetooth, Firewire, or any kindof communication port. When powered down, if the holder is connected toany external device, it has a capability to automatically power itselfup. It also has a capability to power itself down when an externaldevice is disconnected. There are two types of connections for theholder:

One connection is an external storage type of connection. Such aconnection is made with a computer, or other device, called host, thatis capable of displaying graphics and has an adequate user interface.While connected to a host device, the holder behaves as an externalstorage device. A user of the host device can browse through the holderfile system, copying, viewing, and editing files previously collected bythe holder. The software residing on the host is capable of converting,displaying, printing, and editing on the host screen files stored on theholder or copied from the holder to the host. While connected to thehost, the pen can also behave as a real-time input device.

The other type of a connection is with a portable internet or modemenabled device, such as a cellular phone. Upon detecting such aconnection, the pen holder automatically initiates transmission of allthe data previously collected to be sent as e-mail or fax.

The optional LCD display notifies a user of the pen's status, forexample, with respect to connections and downloads over Internet-readycellular phones where communications are not reliable. This display canbe mounted on top of the holder. If an LCD is used, the LED may not beneeded.

A single three-color (green, yellow, red) LED 170 (see also FIG. 18)indicates normal acquisition of writing data, downloads to a PC and overcellular phone, and monitoring of battery and memory status.

Pen, clip, and inkwell switches 141, 143, and 145 are used to controlthe ASIC and a reset switch 147 is used to reset the ASIC.

FIG. 19 shows a state diagram of the states of operation of theinvention. Shaded blocks indicate study states. Clear blocks indicatetransition states. Among other things, the figure identifies the mannerin which the multicolor LED is used to indicate the state of operation.

At power up, the green light blinks as many times as there are pages inmemory. The green light is not on at power up when the memory is empty,and stops blinking after 30 seconds or sooner if the user starts towrite.

During writing, when data acquisition is proceeding properly, the LED ispale green. The green light goes off for faulty acquisition triggeredby, for example, obstructed light, a pen that is off the writingsurface, or a discharged battery.

Low battery status is indicated by a blinking yellow light when nowriting is occurring. However, when writing, the yellow light blinksintermittently with pale green if the battery is low.

Memory nearly full status is indicated by a double-blink of the yellowlight when writing is not occurring and a yellow light blinkingintermittently with pale green when writing is occurring.

Download status (which may start independently whether the pen is in orout of the holder or ink well) is indicated by a bright green lightafter successful download. Blinking green, signifies that download is inprogress. When no service is available for downloading or the downloadsignal is week, a red light blinks. The red light double blinks for anInternet problem, for example when a server is down. A triple blinkingred light indicates a wrong setup for communication including a wronguser ID or server address. This requires a code sent back to pen fromserver after unsuccessful match of data from pen with account ondatabase.

Battery recharge status is indicated by a solid green light after asuccessful recharge and by a multiple blinking green light when theholder is plugged into the AC adapter and charging. A combination ofsignals from the battery monitoring circuitry and the fast charge signalfrom a charger (high when not charging) can identify the state, whethercharge in process or trickle charge.

The pen can be used during recharging. If the pen is removed from theink well and is used during recharging, the yellow light is replacedwith all normal indicator lights described above.

Writing to flash status is indicated by a continuous yellow light.

All errors are reset by activation of any of the two pen or ink wellswitches mentioned later. The only exception is when a download wassuccessful, and the user started writing. Then the constant bright greenlight will switch to a pale green light.

In sleep mode, all trouble indications, low memory, and low batterycontinue as in the normal mode. All download troubles stay on also.

If the ASIC needs to indicate low memory or low battery conditionsduring power up, the power up indications take the priority. Then thetrouble indications are displayed up after a 30 second timeout. If theASIC needs to indicate low memory or low battery conditions duringdownload, the download indications take precedence. After a reset ofdownload status, the trouble indications are displayed.

Of the four switches on the holder, the clip switch 141 indicates thatthe clip is being opened and closed as a way to notify the circuitrythat the user is beginning a new page. The pen switch 143 indicates whenpen is in or out of the holder. An ink well switch 145 indicates whenthe pen is in or out of the ink well. The reset switch 147 is hidden butaccessible through a hole in the bottom using a paper clip.

The pen switch and the inkwell switch indicate when the pen is in theholder or the inkwell and remove power from the data acquisition andstorage electronics when the pen is in the holder or the inkwell. Thepen switch also opens new files (or pages) on activation, while the inkwell switch does not.

The clip switch indicates when the clip is activated, as well as a newpage and beginning of a new file (each page is a file).

The reset switch resets the ASIC if the software freezes. The switchesare normally ON as follows:

Pen ON when the pen is out of the holder.

Ink Well ON when the pen is out of the ink well.

Clip ON when the clip button is released.

Reset ON when switch is depressed.

The holder also includes a miniature connector for USB and RS232interfaces as well as an antenna for use with Bluetooth or otherwireless technology. The USB and RS232 connector are also connected tothe wake-up power circuitry so that pen holder can power itself up whencable is plugged into the miniature connector.

Angle signals generated by the sensors are processed by the ASIC andstored in flash for later transmission to other devices such as cellularphones, PDAs, and PCs (not shown) where they can be used for handwritingrecognition or to capture drawings. The transmission can be done using,for example, USB, RS232, IrDA, or Bluetooth protocols.

File System

The flash memory is structured as a FAT (file allocationtable)-compatible file system, where each file represents one page ofhandwritten information. Each file has a unique name of 12 characters,including 3 characters of extension and a separating “dot”.

Data File Creation

When a user brings the pen into writing mode by taking the pen out ofthe holder, or by pressing the new page clip button if the pen isalready in the writing mode, a new file is created, and the subsequentwriting is saved in a new file. If the user does not actually do anymore writing after new file was created, the newly created file isdeleted, and the next time pen is brought to the writing mode, the samefile name will be reused.

During data acquisition, uncompressed data is stored in a temporarybuffer in SDRAM and compressed by a data store task before being storedinto a file in flash memory. Each page is stored in a separate file. Aprevious page is compressed before new page acquisition is started.

Data File Format

We use a binary compressed format based on a variable rate Huffmanencoding with cubical appoximation. Such a format comprises encoded datacoordinates and timestamps.

Before being compressed, the file has the following format:

The file is structured in four byte segments. Each segment correspondsto either one pixel or one timestamp. Each pixel has a most significantbit (MSB) of zero, and consists of two 15-bit numbers that are the subcoordinates of corresponding CMOS sensors. Timestamps are distinguishedby a MSB of one, and can store either full date and time of the nextpixel (called full timestamp), or incremental counter of pixels sincethe last full timestamp.

Each file begins with the full timestamp. An incremental timestamp isinserted in the end of every written stroke. Because all pixels arescanned evenly in time, such a combination of timestamps enablesefficiently recover the whole history of handwriting in the futureprocessing.

Downloading of Data

When the holder is connected to a PC using a USB cable, the PCautomatically recognizes the holder as a PC-compatible USB device, andthe contents of the holder file system becomes visible for the PCthrough the PC-file system extension. The user can browse through it andview the files using a handwriting viewer.

When an RS232 cable is connected between the holder and, for example, acellular phone, the holder automatically powers itself up, and startstransmission of data files from the memory of the holder to the phone.IR transmission of data to the phone could also be done.

The data is sent in the compressed form to the server and is kept thereuntil requested for an addressee. Then it is decompressed and translatedinto one of the following formats: .tif, .pdf, .gif, .ps specific toe-mail or FAX service.

Sensor Signal Preprocessing

In some examples, a preprocessor (not shown) can be used for backgroundcancellation, and storage into flash memory, while the ASIC processorperforms all communication and I/O functions. The preprocessor can beimplemented as a programmable device such as PLD, FPGA or digital ASICor a DSP. In this example, a frequency multiplication is performed togenerate a high-frequency pixel clock and a clock for the preprocessorfrom the pen LED modulation frequency that is recovered by the PLL.

The second processor can be a processor of another portable device suchas a cellular phone or PDA.

Data Acquisition

Position data is collected in a succession of samples spaced 10milliseconds apart to adequately capture writing motion at a typicalspeed of 5 cm per seconds for a resolution of 0.5 mm. The ASIC operatesas a master, generating the clock and all necessary signals for thesensors.

The sensors in the holder use the pixel clock from the ASIC. A framesignal is generated by each sensor and read back into the ASIC. Thus,the LED pulses from the pen and the signal acquisition performed on theholder are not synchronized in some implementations. In other examples,the data acquisition is synchronized with the pen modulation frequency.Synchronization significantly improves angle resolution.

In each sampling cycle in which the pen position coordinates areobtained, data is captured from both sensors. One version of thebackground cancellation algorithm (asynchronous with pen) requirescapturing three consecutive frames at each sensor. An additional frameis used by the ASIC architecture for sensor reset.

To minimize any skew between coordinates from the two sensors, themultiplexer data acquisition alternates between the two sensors for eachpixel.

Operating the A-to-D converter at a sampling rate of 1.2 MHz maximum andalternating between the two sensors allows for a pixel samplingfrequency up to 600 kHz. Each CMOS array has 1024+4 pixels, whichproduces a frame rate of approximately 600 Hz. A slower rate of 300 Hzmight be used to achieve more pixel exposure to light and accordinglybetter signal-to-noise ratio.

Each sensor operates in a mode in which each pixel is reset after beingread into A/D converter.

The IR LED duty cycle is 50% out of three frame intervals. For that dutycycle, the LED frequency cannot exceed 200 Hz.

For purposes of cancellation of background noise and low frequencyinterference without synchronization, three data frames of 1024 pixelsare required, as described below.

In addition to the main analog output each CMOS delivers END_FRAMEsignals. From each CMOS the acquisition cycle for each of the threesequential frames of data is started by the END_FRAME signal, whichcoincides with the last pixel of the frame. Each A-to-D conversionoccurs on the falling edge of the PIXEL_CLOCK pulse. The total number ofpoints is essentially (1024+4)*3, where 1024 is the length of the CMOSarray, 4 is the number of clock pulses between the END_FRAME signal andthe beginning of the next frame, and 3 is the number of sequentialframes needed to implement background compensation.

From the acquired waveform, the ASIC extracts three arrays, eachcorresponding to 1024 pixels. The arrays must be correctly aligned sothat the i-th element in each of them corresponds to the i-th pixel ofthe CMOS.

Let us call the arrays A1, A2, and A3. Background compensation is basedon the fact that the LED in the pen is modulated with a frequency equalto ⅓ of the frame rate and with a 50% duty cycle. To achieve backgroundcompensation, the following calculations are performed element-wise onthe arrays: A12=abs(A1−A2); A23=abs(A2−A3); A13=abs(A1−A3). Then arraysA12, A13 and A23 are added element-wise to form a new array called A.This array A is 1024 elements long and carries the beam information withthe background removed.

To reliably get rid of the large peaks appearing in the pixel waveformduring the END_FRAME pulses, subarrays shorter than 1024-elements longcan be extracted, for example, three 1020-element long subarrays, thatstart at pixels 3, 1032 and 2061 (base 0).

The readouts of the two sensors are digitized simultaneously (orquasi-simultaneously when using only one A-to-D converter with amultiplexer).

Finding Peak Position Along CMOS Array with Subpixel Resolution

Determining the angle of receipt of the light at each of the sensorsdepends on determining the pixel location of the peak light intensityalong the array of the sensor. The algorithm to find peak position withsubpixel resolution uses two parameters: T, the intensity threshold involts and W, the window width in pixels. Typical values of theseparameters are T=0.1 V and W=15.

As an initial step, the peak value and its index in the array A arefound, call them Amax and M. If either of the two Amax values(corresponding to the two sensors) is smaller than T, then the point isdiscarded. In that instance the LED is considered to be off with the pennot touching the paper. If M<W/2 or M>(1024−W/2), the point is discardedas being too close to the edge of the field of view.

From A, extract a W-element-long subarray starting from element M−W/2.Find its fractional center of gravity as follows: create an array ofrunning sum of elements of the extracted subarray (call it S). Take thevalue of its last element. Divide it by 2. Find the fractional index ofthe position of this value in S using linear interpolation/lookup. AddM−W/2 to this value. This will be the fractional index of the center ofgravity of the beam in the original 1024-element array. Invert its signand add 512 (in the case of an A that is 1024 elements long or 510 inthe case of an A that is 510 elements long). The result, P, is thefractional position of the beam with respect to the axis of the sensor(in pixels).

The use of a subpixel algorithm permits an increase of the pixelresolution by a factor of 8 to 10.

Calculating Light Source Angle with Respect to Sensor Axis

As a result of the previous calculation, we have the angular position ofthe beam for each sensor (in pixels). We call them Pleft and Pright(looking at the sensors from the pen point of view). We recalculate thePs in radians based on the sensor geometry. In one example, the pixelpitch L=7.77 microns, the distance from the lens to the CMOS is D=4800microns (typical), the refraction coefficient of the lens material is N(1.5 for glass, 1.4 for plastic, 1.8 for SF6). Parameters, distance D,index of refraction N, and horizontal offset, Off, will be adjustedusing calibration data for correct mapping of writing.

Then the angle (in radians) is calculated asF=arcsin(N*sin(arctan((P*L)/D))).

As illustrated in FIG. 22, the following parameters are required forcalculating the light source position in Cartesian coordinates:

Sensor convergence angle (toe-in) C (radians), typically 30/57

Base B, the distance between sensors (mm); typically 150

Left sensor: Kleft=tan (C−Fleft)

Right sensor: Kright=tan (C+Fright)

X(mm)=B*Kright/(Kleft+Kright);

Y(mm)=Kleft*X.

Criteria for Accepting a Point as Valid

Points are stored as coordinate pairs (X,Y). When saving points into thememory, coordinates are saved continuously, except as follows:

If the signal is found to be below the threshold (as described above),then a marker (a pair of unique values) is written into the memory, forexample (NaN,NaN) which will signify later that the pen was lifted (NaNstands for not-a-number as defined in the IEEE arithmetic standard).After that, no new points are added to the file until the signal isdetected again. This approach allows the pen to tell the playbackprogram exactly where to interrupt the restored trajectory line.

If the signal is significant, but the pen position did not changesignificantly as compared to the previous position, then no new point isadded to the memory, but unlike the case of no signal, no markers arewritten to the memory. The size of the move squared is calculated as(X1−X0)²+(Y1−Y0)². The typical value for the significance of the movesquared is 0.04 mm².

No timestamps are included in a file because this information is notrequired for restoration of the pen trajectory.

Coordinates are stored in the temporary buffer and are compressed onlybefore storing in flash memory. Each page is stored in a separate file.Therefore, there is no need for an end of page mark. Full time stampswill be inserted before the first valid pixel. All other timestamps on apage (file) will be incremental and inserted whenever the pen is liftedoff the paper. Only one time stamp is inserted regardless of how longthe pen was off the paper.

Sleep Modes

When the pen is taken out of the holder or the ink well for writing, theASIC turns on in the sleep mode and waits until an optical signal isdetected from the pen.

When the holder is awake and it detects that writing is interrupted fora predefined period of time, the holder returns to the power-savingsleep mode. The ASIC enters sleep mode by reducing its normal 48 MHzclock frequency to 750 kHz. SDRAM update refresh rate also changesaccordingly to keep data intact.

The holder power is almost entirely off when the pen is inside theholder or in the inkwell. RS232 receiver and USB monitoring circuitsconsume very little standby current. These circuits wake up and enablepower to the rest of the electronics on detection of active levels forRS232 or USB, when connected to a cellular phone over the cable or byUSB cable to a PC. The pen holder is completely off when the pen isinside the holder.

In sleep mode, the only function of the holder electronics is to watchfor a WAKEUP input from photodiode and associated PLL circuitryindicating that the pen is active. In sleep mode, the pen consumeslittle power between the time intervals when it checks the photodiode.

During writing, the pen transmits modulated IR pulses. The pulses aredetected at the holder causing the PLL to wake up the processor, whichstarts normal acquisition mode as soon as the ASIC switches back to the48 MHz system clock.

Phase Lock Loop (PLL)

When the modulated IR light from the pen is being detected, themodulation clock of the pen LED (represented by 1 kHz bursts in theoutput light) is extracted using PLL circuitry 132 tuned to themodulation frequency of the IR light.

All acquired data is initially stored in SDRAM 134 using DMA. The updaterate of the SDRAM remains unchanged when going from acquisition mode tosleep mode. The memory requirement is 1 Mbyte for 50 pages of compressedor 10 pages of uncompressed data. The 5:1 compression algorithm musthave fast and computationally simple coding with no limitation ondecoding.

The acquired data is initially stored in SDRAM during writing. When thepen is returned to the ink well or the pen, or when the new page switch136 is activated, the ASIC writes all data from SDRAM into flash memory138. Only a short time is needed to write a full page of hand-writtentext data into flash. The transfer is indicated to the user by lightinga yellow LED 140 on the holder.

8 Mbit flash memory stores compressed files representing a maximum of 50pages of handwritten text. The compression algorithm allows at least6-to-1 compression without observable distortion of text.

Power for the Holder

The holder is powered by two AA NiMH batteries connected in series toprovide 3.0V. When the pen is in the ink well or the holder, the pen'sthree NiCd batteries are recharged by a trickle current. The penbatteries have a large capacity and are almost never rechargedcompletely. The trickle current charging is enough to maintain thebattery charge. A special mode is provided when the pen and the penholder are both in the charger to charge all the batteries including thepen batteries with the full charging current.

Battery life is ten handwritten pages or a week of average use withoutcompression of data for storage in memory. An average user may write 2characters per sec, or 120 char/min, or 7200 char/hr. The averagehandwritten page is approximately 700 characters. To produce ten pages,the battery must work for 5 hours.

When connected to a USB port, the holder can get power from the USBhost. The charge on batteries is maintained at a high enough level tostart the circuitry prior to switching to USB power. Power from the USBconnector is provided only after the ASIC establishes communication overthe USB link and notifies a PC on the other end of the USB link that theconnection is “high power”. In response, the PC provides up to 0.5 A.Battery charging current is set at 0.4 A and is monitored to switch thecharger into trickle charge.

The holder circuitry is activated when the pen is taken out of theholder or the ink well. Some holder circuitry, like the RS232 driver andwake-up power circuitry take power directly from the battery. Othercircuitry takes power from a 3.3v supply generated by an on-boardswitching regulator from the battery voltage of 2 to 3 volts. Whenconnected to a USB link, the 3.3v is generated from USB power.

5v is generated for the analog circuitry from the 3.3v supply.

Synchronization of Pen and Holder

Synchronization of the pen and the pen receiver can produce a bettersignal resolution and correspondingly better angle resolution andresolution of writing.

As shown in FIG. 20, for synchronization, the pen produces periodicbursts 401 of higher frequency pulses, such as pulses at 1-10 kHz(suggest we show some timing diagrams) that can be easily detected bythe PLL. The PLL will detect not only the actual modulation clock butalso its phase, which enables a signal to be generated to start dataacquisition and synchronize it with the pen LED.

As shown in FIG. 21, the control signals, LED_ON and LED_OFF, triggersignal acquisition. In such a case, only two frames will be required forbackground cancellation, one for the IR signal when the pen LED is on,and the other, when the LED is off. For a CMOS sensor, a shutter mode isprovided that resets all pixels at one time.

Having only two frames per sample raises the sample rate and resolutionand may allow the processor to go into idle mode in between the samplesto save power.

Use of 2-D CMOS Arrays

Vendors to manufacturers of digital sensors produce small power-savingsensors and sensors along with the image processing circuitry that canbe integrated into the pen on paper or 3-D pen applications.

3-D positioning of a light spot is possible using two 2-D photo arrays.Projection of a point of light onto two planes defines a single point in3-D space. When a trajectory of 3D positions is available, motion of anIR pointer-pen can control a 3-D object on a PC screen. When the penmoves in space, it drags or rotates the object in any direction.

Slave Mode

In other implementations, using the ARM7 based ASIC in a slave mode, theDMA can handle the data acquisition, but the vertical synchronizationsignals are provided by the pen light detection circuitry (PLL).

Two Analog Channels Alternative

Two separate channels can be used for analog signal processing andA-to-D conversion. Such an implementation could use more economicalparts that do not require fast settling times, frequency bandwidth andslew rate.

Frame Varying Alternative

CMOS sensors have a limited dynamic range. Although an adjustableelectronic gain may be used for both CMOSs simultaneously after theoutput of the CMOSs, this arrangement may be not be ideal, for tworeasons.

First, the signals for the two CMOSs may be different in magnitude whenthe pen is being moved in certain areas of the paper, so changing thegain for both may fix one signal while degrading another signal to anunacceptable level. To get suitable signals across the page, it isuseful to have separate gains for each CMOS. Second, using an electronicgain does not do anything to prevent saturation of the actual CMOS,which is unavoidable with the area that the sensors must cover.

The gain of the CMOS can be changed by changing the exposure rate foreach CMOS independently. As shown in FIG. 21, the pen transmission rateremains 100 Hz, while the frame rate 601 of the CMOS is shifted among300 Hz, 600 Hz, and 1200 Hz. At 300 Hz, the background cancellation isstraightforward. For 600 Hz, the algorithm uses every other frame(frames 1, 3, and 5). For 1200 Hz, the algorithm uses every fourth frame(frames 1, 5, and 9). The pixel rates are 300 kHz, 600 kHz, and 1.2 MHz.Changing the frame rate can be accomplished by the ASIC without anyadditional hardware.

Each CMOS may be connected directly to its own ADC or both could beconnected to one ADC that would be able to handle 1.5 megasamples/secand have a 4V reference voltage. The ADCs then may feed into a digitalmultiplexer so that the signals can be fed into the ASIC.

PSD Based Approach

Instead of CMOS arrays, two PSDs may be used to detect the IR light fromthe pen. Each PSD determines the angle between the page and a line ofsight between the pen and the PSD. The two angles from the two PSDs andthe distance between the PSDs are sufficient to compute the location ofthe tip of the pen.

Even with IR filters, ambient light will introduce errors in PSDpositioning measurements. To reduce the errors, the IR light at the penis modulated to generate pulses at a modulation frequency and with a 50%duty cycle, as described above.

Two analog techniques may be used to discriminate the PSD signal that istranslated into the angle for use in triangulation.

In one approach, called synchronous demodulation and used ininstrumentation electronics, the incident synchronous light pulses arechopped at the light modulation frequency, and opposite gains (+1 and −1respectively) are applied to those signals, depending on whether the LEDis on or the LED is off. This allows for subtraction of backgroundnoise. Then the signal is integrated using a time constant that it isresponsive to the signal variations on one hand and averages out noiseon the other. In one example, the modulation frequency could be 3 kHz,and the pulse amplitude could be ILEDpeak=Xma.

A second approach to discrimination uses a sample and hold technique.The shape of the optical signal has a 50% duty cycle at the 3 kHzmodulation frequency, as before, but also has a significantly strongershort pulse imposed on the modulation frequency. The modulationfrequency is discriminated using a PLL and is used to trigger the sampleand hold circuitry, while the strong optical pulse is actually sampled.The pulse amplitude is ILEDpeak=Xma and the pulse duration T=Y usec.

PSDs are extremely accurate in sensing and measuring the position oflight on their photosensitive surfaces. They are inexpensive and requirevery little power consumption. The PSD implementation is also simplerthan the CMOS one.

As shown in FIG. 23, current-to-voltage transformation is done on eachof four channels, two for each PSD. The four analog signals pass throughlow frequency filtering 605, synchronous detection 607, integration 609,and digitization 611 by microcontroller 613 (12-bit A/D converters).A-to-D conversion is performed at a 100 Hz sampling rate. The processoris active when the pen is making a trace on paper. The processorperforms signal acquisition and periodic storage into flash memory. FIG.24 shows a system timing diagram. When no trace is being made, themicrocontroller enters the idle mode, and after an arbitrary period oftime, the sleep mode.

The microcontroller is awakened from idle mode or sleep mode by eitheran interrupt or polling (TBD) of the following inputs: one of the fouranalog channels, when the signal at its modulation frequency exceeds acertain threshold of the comparator; an interrupt from a USB port whenpresence of activity from a host is detected; one of its key buttons ispushed.

When the RAM becomes full or/and the boundary of a flash memory page isreached, the processor writes data from RAM to flash memory. Ifacquisition continues and the page is full, the microcontroller startwriting to flash. However, most of the flash operations should be doneduring idle cycles when there is no writing.

Each PSD has two channels of analog signal processing. Each channel hasa current-to-voltage converter whose output is AC coupled into the firstgain amplifier. The signal is chopped with the modulated frequency ofthe pulsing IR LED (on pen), currently 1 kHz.

When LED emits light, the chopper has a gain of +1. When there is nolight, the gain is −1, therefore the signal is synchronouslydemodulated. The last stage is an integrator, whose output is close toDC. More precisely, it is a saw-tooth waveform due to charging anddischarging of the integrating capacitor in the feedback of theamplifier.

The A/D converter, either a PC-based DAQ or an A/D of themicrocontroller, samples the output at specified time intervalssynchronously with the modulation frequency to cancel errors due tosaw-tooth waveforms.

To use all 12 bits of the A/D converter resolution, a dynamic change inreference voltage for the converter is used. The microcontroller alwaysstarts reading the A/D channels with the highest range and then dividesit in half until the range is the most optimum for the signal.

The chopper amplifier uses a replica of the modulation frequencydetected with an analog circuitry on each channel (four channelsaltogether). This signal is taken after the second gain stage, processedfor detection of signal transitions, and then the recovered modulationpulses pass through OR gate to drive the chopper amplifier analog switchto change its gain between +1 and −1.

Phase Shift Using Photo Diodes, Rotating Pen Tip

As shown in FIG. 25, if a rotating light source 617 is used at the tipof the pen, it is possible to measure the phase difference among signalson three photodiodes 619, 620, 621 on the holder to find the penposition.

The rotating light on the pen tip can be realized using several (e.g.,eight) LEDs 623 that are triggered at times spaced apart by T/N, where Tis the overall time period of the LED cycle, and N (e.g., 8) is thenumber of LEDs.

The signal source is at some location on an X-Y plane. Two signaldetectors 619, 620 are located at two other fixed locations on the sameplane. If the signal source has a radiation pattern such that the signalradiated in the positive X direction is in phase quadrature to thesignal radiated in the Y direction (spatially rotating at the signalfrequency), and the propagation delay is negligible compared to thesignal period, then the angle A1, formed by two intersecting lines 637,639 drawn from the detectors to the signal source will be the same asthe phase difference between the signals measures at the detectors.

If a third fixed location detector 621 is added, then a second angle A2will be formed as three lines intersect at the signal source. Again, theangle A2 between the lines at the intersection will be the same as thephase difference of the signal measured between the detectors. Byapplying some basic trigonometry, it becomes possible to find thelocation of the signal source in the X-Y plane by knowing the fixedlocations of the detectors and measuring the phase differences of thesignals at the detectors. If the three detectors are arranged in astraight line with equal distances between them, the computation becomestrivial.

Referring to FIG. 26, the calculation of B and A angles based on theangles measured by the sensors, “a” and “b” is as follows:Having: a/A=d/R  (1)and b/B=d/R  (2)and B+A+b+a=180°  (3),

from basic geometrical theorems,We get: B/A=b/a,  (4),and accordingly B=A×b/a  (5)A=B×a/b  (6);

Now plugging (3) into (5) and (6) we get:A×b/a+A+b+a=180°  (7) andB×a/b+B+b+a=180°  (8);

We solve them for A and B:A=a×(180°−b−a)/(a+b)  (9)B=b×(180°−b−a)/(a+b)  (10)

The rotating light on the pen tip can be realized using several (e.g.,eight) LEDs 623 that are triggered at times spaced apart by T/N, where Tis the overall time period of the LED cycle, and N is the number ofLEDs. For example, eight light emitting diodes (LED) could be arrangedin a circle pointing outward, spaced 45 degrees apart and driven by ansignal oscillator with a 45 degree phase difference between adjacentLEDs.

As shown in FIGS. 27 and 28, the three detectors 641 could be PositiveIntrinsic region Negative (PIN) diode optical detectors driving a signalprocessing chain 642 consisting of a trans-impedance amplifier 643 and ahigh gain limiter 645 to remove any amplitude modulation in the detectedsignals.

Phase detection could be accomplished with two edge-triggered one bitUp-Down counter type phase detectors 649, two binary counters and aclock running several decades above the signal frequency. If thecounters are connected such that they count up with every clock cyclewhere the one sensor leads the phase of the other and count down whenthe phase lags, and a third counter is set to count up continuously,then a microprocessor can periodically read and reset all the counters,scaling the reading from the two counters connected to the phasedetectors (dividing by) by the reading from the continuously runningcounter. This number is the phase difference (in gradients) between thethree sensors and as such, the angles between the intersection of thelines from the sensors to the source. It is then a trivial task tocalculate the location of the source relative to the sensors.

Pen Light Activation Switch Alternatives

Different pen light activation methods can be used, including conductiverubber, pressure sensitive materials or strain gauges.

Pressure sensitive material allows for a variable pressure threshold andcoordination of the switching point with the ink flow. This wouldprevent loss of data when the ink is making a trace while the pen is notactive yet. Most ball point refills release ink at 20 to 30 gf+/−30%,while an off-the-shelf switch activates at 50 to 100 gf and +/−40 gf,for example, making a reliable coordination of ink flow and data captureimpossible. Special refills can be also designed to prevent ink flowbelow 50 gf that might enable the use of off-the-shelf chip switches.

Pen Optics Alternatives

Other approaches for emitting light from the tip of the pen arepossible. Optical fibers could be used to collect light from an LED andemit it in a 360° pattern around the tip of the pen. Individual LEDchips could be located around the tip of the pen and emit light througha half reflective lens/window, such that 50% of light is emitted and theother 50% is reflected internally to be mixed with other light,ultimately producing uniform 360° illumination. Light could be mixedfrom a single LED using special rings that redistribute the light foruniformity.

Passive Pen Alternative

The pen may be completely passive if the IR light source is located nextto the sensor. A reflective surface would be provided near or at the tipof pen. The sensors would see reflection of IR light from the tip of thepen and compute angles as described above.

The tip of the pen must be reflective only when pressed against paperand ink is forming traces. Otherwise there will be erroneous traces indigital form with no corresponding traces on paper.

Activation of the reflective mechanism can be mechanical or electrical.In a mechanical implementation, pressure on the tip will open up asheath and expose reflective surface around the tip. In a electricalimplementation, pressure on the tip will activate liquid crystals orother photo technology that will make that material reflective to light.Reflection from other objects, like fingernails and rings, can behandled by using polarized IR light.

Passive Pen Holder

Conversely, the holder may have two reflectors, while the pen both emitslight and receives reflections. The sensing element on the pen could bea 2-dimensional PSD or CMOS array. If flat 2-D sensors are used, the penwould not be omni-directional, but it would be possible to make a customcircular 2-D sensor that would have 360° coverage.

Keyboard and Mouse Replacement Architecture

The pen described above can be used to replace standard PC input devicessuch as a mouse and a keyboard.

When used as a replacement for a keyboard or a mouse, the sheet ofpaper, plastic or other flat surface, may bear a printed keyboardpattern that will serve as a keyboard and mouse pad for, e.g., PC's,handheld computers, and cellular phones.

Users today are, for the most part, limited to keyboards, keypads orstylus input on screens when inputting data into PC's, handheldcomputers or cell phones. Keyboards are efficient and convenient whenthey are full size but do not lend themselves to portable devices suchas palm computers and cellular phones. Cell phone keypads, whileefficient for dialing phone numbers, require excessive keystrokes whentrying to generate ASCII letters and symbols, making any type of datainput a very tedious and time-consuming process. Styli on screen inputwith palm devices requires either that the user use unique writingstyles such as “graffiti” in order to minimize the amount of handwritingrecognition needed on the device or that the user tap on small virtualkeyboards represented on the screen. Both styli approaches often resultin incorrect input, which limits the functionality of these devices.

An electronic pen can be used in a mode that provides a highly reliablemethod for inputting text characters in addition to recordinghandwritten images and lines. A sheet of paper or any other surface(with or without a printed pattern of a keyboard) is all that users needto type with a pen.

The spatial transcription capabilities of the electronic pen togetherwith the keyboard template are used to substitute for the mouse orkeyboard.

The paper keyboard can be multiple sizes based on the user's needs. Thesize can range from an 8½×11 sheet of paper to the size of a cell phonecover. The user first selects the size of keyboard he desires and thencalibrates by touching the pen to specified characters on the keyboard.To type a message, the user touches the tip of the pen on theappropriate keys. When the pen touches the square area of the paper thatcorresponds to a certain letter the location of the tip of pen iscomputed and the designated letter is determined. This approach allowsthe user to generate text on a computing device with an electronic penwithout any dependency on handwriting recognition software.

This approach is an improvement over the built in keypads, softwarekeyboards or styli on screen input approaches currently used onever-shrinking personal appliances. The paper keyboard allows the userto enter messages on handhelds and cellular phones faster and morereliably than with alternative approaches. The paper can also be used inother modes to record drawings and handwritten notes and images. Whendone, the paper keyboard can be discarded or folded for future use.

In addition to characters, the keyboard can also contain shortcut keysand function keys that enable more efficient interaction with a smalldevice. Short cut keys can minimize the number of keystrokes required toenable commands. Short cut keys can be customized based on the type ofdevice being input into.

The keyboard can also incorporate a section that serves as a paper mousepad. Using the electronic pen's spatial transcription capabilities, theuser can move the pen within a designated square space on the paper thatin turn moves a pointer on the screen of a device. The paper mouse thusserves as an alternative to keys and styli on screens as a means fornavigating on the screen of a handheld or cellular phone.

The paper keyboard also enables flexibility in inputting foreigncharacters. Keyboards can be created for several languages such asJapanese, Korean, Spanish, French and Russian. A user can simply printout a new keyboard if they desire to input a different alphabet.

As shown in FIG. 29, to type a message, the user touches the tip of thepen 701 on the appropriate keys 703 printed on a paper keyboard 705. Thepositions at which the pen is touched are tracked by tracker 707,converted to text and then sent to a portable device such as a cellularphone 711 or a palm computer 709.

The keyboard can be folded or discarded after the use.

In other implementations, there is no printed paper keyboard; rather thetracked motions of the pen can be used through handwriting recognitionto derive text, commands, and drawings as the pen is used for writing onany surface.

In one implementation of this approach, the mouse and keyboard cancontinue to be used and the pen serves as an alternative. The pen canoperate in either a “pointing device” (mouse) or a “character input”(keyboard) mode. The mode can be selected by a dedicated hardware switchor button, on a pen holder or pen, or by sending a command from PC to aholder.

In mouse mode, pen operation is indistinguishable from that of asecondary (USB) mouse. It is a relative positioning pointing devicemoving the cursor on the screen. In keyboard mode, pen input can bereceived by a specially designed application using an availablecharacter recognizer to convert graphical input (strokes) intocharacters. Other applications are not aware of the pen presence andcontinue to operate using the regular (legacy) keyboard.

In another approach, the pen is the only input device to the system. Inthis case, a software driver stack is modified to provide keyboardfunctionality system-wide. Mouse-mode operation is not affected and isidentical to the first-described approach. When operating in keyboardmode, pen input is recognized by available handwriting recognitionsoftware built into a keyboard filter driver and then delivered tosystem input queue in a similar way to traditional keyboard input.

This second approach requires a platform usability model and (mostlikely) modification of certain system components such as BasicInput/Output System (BIOS).

Both approaches raise human factor and usability issues. In particular,there are two basic approaches to handwriting recognition: discrete(single character at a time) and continuous (word, phrase or page at atime). In the former case, the user must continuously rely on computerscreen output for feedback. This may be awkward, because the handwritingprocess must be constantly interrupted by looking at the computer screenfor feedback. In latter case, the user must only look at the screen oncein a while, when a writing unit (word or phrase) is completed andcorrect it as necessary.

Switching from mouse to pen input mode could be done by retractable penrefill action. When the refill is inside the pen (pen cannot write), itis used as a mouse. When writing is activated, the pen acts as akeyboard.

Other embodiments are within the scope of the following claims.

The holder need not be of the kind that includes an inkwell as describedabove but can be any kind of device that can hold the sensors. Theholder can be a simpler pen cap, as shown earlier, or could be any otherkind of device whether or not it mates with or caps the pen and whetheror not it includes a clip or not. The holder could be incorporated intoa clipboard or a notebook, for example.

The light in the pen can be fiber optics that deliver light to the tipand convey it in all directions around the pen in a disk-like pattern.

1. Apparatus comprising: a writing instrument including, an elongatedhousing configured to be hand-held, a light source in the housing, and alens in the housing configured to receive light from the light sourceand convey the light through a free-air path to optical sensors spacedfrom the writing instrument, the lens being configured to enable lightto be directed parallel to the writing surface no matter what theorientation or position of the writing instrument to the writingsurface.
 2. Apparatus comprising: a writing instrument including, anelongated housing configured to be hand-held, and a light source in thehousing, the light source comprising an array of light sources arrangedaround an axis of the writing instrument and configured to emit light ina direction normal to the axis.
 3. The apparatus of claim 1 in which thelens is configured to internally reflect and concentrate the light andto emit it by reflection from a reflective external surface of the lens.4. The apparatus of claim 3 in which the reflective external surfacecomprises a conical surface oriented at a certain degree angle to alongitudinal axis of the writing instrument.
 5. The apparatus of claim 1in which the lens comprises a cylindrical body having an upper surfacethat receives the light and a lower annular surface that reflects thelight toward the optical sensors.
 6. The apparatus of claim 1 in whichthe light source emits light in a direction toward a writing end of thewriting instrument.
 7. The apparatus of claim 1 in which a light guidedelivers light to the tip of the pen and conveys it outwardly in adisk-like pattern.
 8. Apparatus comprising: a writing instrumentincluding, an elongated housing configured to be hand-held, and a lightsource in the housing, the light source being arranged to emit light ina direction parallel to a longitudinal axis of the writing instrument.9. The apparatus of claim 8 in which the light source comprises one ormore LEDs.
 10. The apparatus of claim 8 in which the light sourcecomprises a ring of light sources.
 11. Apparatus comprising: a writinginstrument; a light-source in the writing instrument configured toconvey light to sensors spaced from the writing instrument; and a deviceconfigured to turn the light source on and off in response to a userapplying pressure from the writing instrument to a writing surface, theswitch being configured so that an amount of pressure required totrigger the switch is not so large as to disrupt normal writing motionof the writing instrument on the writing surface.
 12. The apparatus ofclaim 11 in which the writing instrument includes a ballpoint cartridgehaving a writing point and the device is positioned at the opposite endof the cartridge from the writing point.
 13. The apparatus of claim 12in which the device could comprise a switch.
 14. The apparatus of claim12 in which the device could comprise a pressure sensor.
 15. Apparatuscomprising: a writing instrument and a sensor, the writing instrumentincluding a reflective element configured to reflect light received fromoutside of the writing instrument to the sensor for use in trackingmotion of the writing instrument.
 16. The apparatus of claim 15 alsoincluding a mechanism to enable the reflective element to reflect thelight to the sensor when the writing instrument is being used forwriting and to disable the reflective element from reflecting light tothe sensor when the writing instrument is not being used for writing.